Prior art dies for thermoplastic materials used in extruder press units, calibrating or forming units and the like often comprise at least one output gap-defining adjustable die lip, an opposing gap-bounding lip, at least one gap-adjusting element working on the gap-defining adjustable die lip, and a control and adjusting device for controlling the thickness of the extruded plastic product and acting upon the adjusting element.
Extruder press units generally comprise the extruder head connected to a worm feed and the control and adjusting device.
The expression "die" as used herein includes both extruder dies and calibrating or forming dies which impart desired dimensions to a semifinished article.
An extruder may be constructed as a tapered nozzle die for extrusion of plates and foils, for extrusion of sections (structural shapes or profiles), hollow sections, pipes and tubing or for blown-foil manufacture. Commonly one such die unit is equipped with several adjusting elements, which are arranged in rows. To simplify the description the adjusting element is referred to in the singular.
In the known die units, for example as described in U.S. Pat. No. 3,940,221, the adjusting element is a metal bolt whose head is provided with threads and is screwed in a corresponding hole.
The shaft of this bolt is heatable by ohmic electric heat from a suitable heating element. Its thermal expansion determines the gap size adjustment. The heating element is associated with the control and adjusting device and at least partially operates it. It allows adjustment to a particular temperature in a given temperature range and it can be held at that temperature, until control and adjustment requires another gap size adjustment and therefore another temperature for the bolt shaft.
This system has several drawbacks. On the one hand the temperature change of the bolt occurs only slowly because of the physical basis of its mechanism, so that increased adjusting element equilibration times with significant disadvantages must be taken into account.
On the other hand injurious unavoidable temperature inhomogeneities of the bolt shaft affect the precision of the control or regulation, chiefly in the heated die, where undesirable control and adjusting couplings occur. As a result the prior art die unit may not produce a product with the narrow tolerances which are required in modern practice.